1. Field of the Invention
The present invention relates to a sensor configured such that an element casing, housing an element for transmitting and receiving ultrasonic waves and electric signals, is attached to a measuring chamber in which an object to be measured by use of ultrasonic waves is present, as well as to a method for manufacturing the sensor.
2. Description of the Related Art
Conventionally, an assembly of an element for transmitting and receiving ultrasonic waves and electric signals and a casing housing the element has been used as a sensor or an ultrasonic-wave generator. Ultrasonic waves propagate through various media. Since the propagation of ultrasonic waves is influenced by properties of a medium, such as concentration, thickness, and temperature, ultrasonic waves are utilized for sensing such properties. For example, in a known gas sensor, an ultrasonic-wave generator that faces a flow path is used for measuring the concentration, temperature, or humidity of a gas present in the flow path. Such a gas sensor electrically processes a signal emitted from a detecting element and outputs an electric signal corresponding to a property of the gas. Examples of such a gas sensor include a gas concentration sensor used in transportation equipment on which an internal combustion engine is mounted, such as an automobile, for detecting the concentration of gasoline or light oil through utilization of variation in propagation speed of ultrasonic waves. Such a gas concentration sensor is disposed in, for example, a purge line extending between a canister mounted on an automobile and an intake pipe of an internal combustion engine of the automobile, and is configured such that an evaporated fuel gas which contains gasoline flows through a flow path of a predetermined volume formed therein. As the concentration of gasoline vapor varies, the speed of ultrasonic waves propagating through the medium varies. An ultrasonic receiver detects this variation of speed and conducts signal processing so as to output a signal corresponding to the gasoline concentration. Usually, the time required for an ultrasonic wave transmitted from a transmitter to propagate over a predetermined distance and reach a receiver is measured, and the gasoline concentration is obtained from the measured time.
Most detecting elements including that of such a gas concentration sensor cannot directly convert variation in a property of a gas to an intensive electric signal; i.e., electric signals output from these detecting elements are weak. Therefore, in a conventional gas sensor, an electric signal emitted from a detecting element is amplified and processed by means of a dedicated signal processing circuit.
Although weakness of an electric signal is solved through amplification and processing, enhancing detection accuracy has been difficult if noise is superposed on a signal which an element is to detect. Conventional sensors have failed to sufficiently reduce the influence of ultrasonic noise on a detecting action of an element, regardless of whether the sensors employ a single element which serves as a transmitter-receiver for transmission and reception of ultrasonic waves, or a transmitter element and a receiver element that are provided separately. Usually, an element casing that houses an element is formed separately from a measuring chamber in which an object to be measured, such as gas, is present. In some cases, attachment of this gas sensor to a measuring chamber in a fixed condition has involved a phenomenon such that ultrasonic waves propagate through an outer wall which defines the measuring chamber, and reach the detecting element. Some cases where an element for transmitting ultrasonic waves is housed in a sensor have involved a phenomenon such that ultrasonic waves transmitted from the element are reflected from a joint between the sensor and a measuring chamber, and the thus-reflected waves are received by a receiving element and act as noise in measurement.